Temperature and pressure dependences of the Grueneisen constant of a mat of polyethylene single crystals.

The sound velocity of the longitudinal ultrasonic wave at 4.64 MHz in a polyethylene (PE) single crystal mat was measured at various temperatures from 15°C up to 90°C and at various pressures below 5500 atm. The Gruneisen constant, γs, for PE single crystals was evaluated from the pressure dependence of the sound velocity. The γs values at 1 atm increased with increasing temperature and showed larger values than those for the perfect crystals predicted by the theory proposed by Broadhurst and Mopsik. This fact was discussed in connection with the mean square displacements of molecules in PE crystals obtained by X-ray measurements by Iohara, et al. The increase of the Gruneisen constant with increasing temperature was explained by an increase of molecular displacements in the crystal due to lattice imperfections as well as to the anharmonicity of intermolecular vibrations. The γs value at higher pressures showed smaller dependence on temperature. This fact was explained in terms of decreases in the volume of lattice imperfections and in anharmonicity of intermolecular potential with increasing pressure which are probably accelerated by compression of the crystal lattice.